In order to carry out high axial resolution tomography of a biological tissue, there already exists the well known technique of tomography by interferometry with a short coherence length (OCT, for Optical Coherence Tomography). For example, the work by M. E. Brezinski and J. G. Fujimoto can be mentioned, in particular in the article “Optical Coherence Tomography in Non Transparent Tissue”>>, IEEE J Sel Topics in Quant Elect, 5: 1185, 1999. This technique relies on a device of the Michelson interferometer type, which consists of making two light beams originating from a single source interfere, one of which is reflected on a reference mirror and the other on a sample to be investigated. The lighting of the device by a source of short coherence length makes it possible to only obtain interferences with which the light returned by a thickness of the sample produces equality of the optical paths in the two arms, at best half the temporal coherence length in the medium.
When the sample to be investigated is arranged in an aberrant medium, or after aberrant optics, such as that of the eye for example, outward and return beams to and from the sample are affected by geometric aberrations. This results in two significant consequences:                Each point of the source of the device, extended in the case of a full-field system, sees its image in the volume of the sample degraded by geometric aberrations: the illuminated area is much greater, or even multiple if “speckles” flash on the image. This calibration effect in the sample is manifested by a spatial mix of the information fed back, and thus by a loss of spatial resolution. It also manifests itself by a lowering of the lighting level, and thus by a lowering of sensitivity.        On the return path, each point of the sample produces a wavefront, which altered again by geometric aberrations, can only interfere partially with the return beam of the reference arm, by the absence of mutual coherence of the wavefronts. The expected contrast of the interference fringes is reduced by a factor e−σ2, where σ2 is the spatial variance of the phase of the disturbed wave front. This phenomenon is well known by interferometry astronomers, who can only design interferometers with a number of telescopes when the pupils of the latter are coherent, either naturally with a large wavelength, or after restoration by an adaptive optical system. In the case of an OCT system, the loss of contrast is manifested directly by a loss of sensitivity.        
Therefore, an input source point is no longer conjugated at a single point of the sample, even close to the diffraction, and a fortiori even less with the detector arranged at the output, although this is still the case for the beam circulating on the reference arm.
These limitations due to geometric aberrations are an additional difficulty intrinsic to the OCT technique: that is a change of observation distance by movement of a reference mirror must correspond to a change of focussing distance (without additional variation of the path difference) in the sample, without which there will again be loss of contrast.
The result of this is that a tomography system used in an aberrant medium sees its spatial resolution and its sensitivity reduced simultaneously owing to geometric aberrations and focussing changes. An OCT tomography system can, by coupling with an adaptive optical method (OA), see its sensitivity and its spatial resolution improved when it is used in media or with optics generating significant geometric aberrations, a fortiori when these aberrations vary over time. Adaptive optics is a technique for restoring wavefronts, which relies on a measurement of the disturbances of the wavefront and on a closed loop correction of this wavefront via a corrective system. There are various ways to measure a wavefront, and therefore different types of analyser. The analyser of the Shack-Hartmann type is the most used, as is illustrated by the documents U.S. Pat. Nos. 6,299,311 and 5,777,719. Applied to the eye, the measurement of the wavefront is carried out on the beam returning from a light spot imaged on the retina. There are also different types of corrective system, deformable mirrors being the most common.
An OCT+OA coupling has already been envisaged as a three-dimensional (3D) imaging solution for biological media. However, the sensitivity levels currently obtained with systems implementing this coupling are clearly insufficient to envisage an in vivo tomography system for the examination of a human retina, for which the measurement conditions are very difficult given ocular movements.